The present invention pertains to a fiber-reinforced thermoplastic composite material in which the intimate bond between the polymer and the fibers is ensured by a second thermoplastic polymer which is compatible with the first one and facilitates the wetting of the fibers. The said composite material is manufactured by coating the fibers with a second thermoplastic polymer which facilitates wetting, the first thermoplastic coating polymer adhering in the hot state to the said wetting-promoting polymer.
French Patents Nos. 2,414,995 and 2,463,221 teach that composites based on fiber-reinforced thermoplastic polymers can be manufactured, but the thermoplastic coating polymer is always brought directly into contact with the fiber. It was observed that the standard thermoplastics used for this kind of application do not possess sufficient wetting properties to ensure an intimate contact with the fibers. This results in a composite possessing a relatively heterogeneous structure, which necessarily leads to a deterioration of the mechanical properties of the end product. Without seeking to reduce this heterogeneity, it is proposed in French Patent No. 1,578,394 that the organic fibers used as the filler be coated by soaking them in a solution of the said polymer, after which the solvent is removed and the dried fiber system is granulated to protect the said organic fibers. This granulated material is then associated with the same, filler-free polymer to manufacture the composite. This complicated method, which destroys the fibers, does not improve the intimate contact between the said fibers and the polymer at all in comparison with the techniques described in the two preceding patents.
To protect the fibers and to avoid their destruction during grinding, it is also proposed in U.S. Pat. No. 3,863,758 that a sandwich be prepared in advance; which is formed by polymer powder-coated fibers, after which the whole system is then wrapped into films from the same polymer. This sandwich is then molded to form a composite. Since the same standard polymer is used to coat the fibers, the same problems arise again as in the case of Pat. Nos. 2,414,995 and 2,463,221.